Re: ARC and blocks
Re: ARC and blocks
- Subject: Re: ARC and blocks
- From: Marco Tabini <email@hidden>
- Date: Thu, 26 Jan 2012 16:48:55 -0600
On 2012-01-26, at 3:51 PM, Jan E. Schotsman wrote:
> Hello,
>
> This code is given in the "Transitioning to ARC Release Notes" as an example of accomodating blocks in an ARC environment:
>
> __block MyViewController *myController = [[MyViewController alloc] init…];
> // ...
> myController.completionHandler = ^(NSInteger result) {
> [myController dismissViewControllerAnimated:YES completion:nil];
> myController = nil;
> };
>
> Supposedly this avoids a retain cycle. But where is the cycle? At least two objects are needed for a cycle. What is the second one?
ARC has changed the behaviour of __block [1] so that it forces a retain of the value (previously, it forced a variable to behave like a weak property). This (as I understand it) is necessary because otherwise the value would be released at the end of the current method, which is obviously not what you want. Note that, even if you don't specify myController as __block, the variable will still be declared with an implicit __strong attribute.
The key is that, by declaring it as __block, you give your block the option to alter its contents. At the end of the block, you simply force it to Nil, which implicitly releases it, thus breaking the retain cycle. Pretty neat, eh?
HTH,
—Mt.
[1]: See the CLANG docs, section 7.5.
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